bug fix for disabling module instrumentation

Previously, modules would explicitly disable instrumentation when
ever Darshan core called into the module to shut it down. The
issue is that Darshan core starts shutting down well before modules
do, so there is a time interval where modules could be trying to
instrument data while its not safe to do so.
Now, we expose a function (darshan_core_disabled_instrumentation)
that modules can call to determine if its safe to instrument
data.